EP0466993B1 - Method and device for releasing the pre-load initially applied to a mechanism such as an on-board bearing in a spacecraft - Google Patents

Description

The invention relates to a method and a device designed to release a preload initially applied between moving parts of a mechanism such as a bearing on board a space vehicle, when this vehicle is in operational mode.

Space vehicles such as artificial satellites and orbital stations include equipment such as solar panels, antennas, etc., which include mechanisms allowing relative movements between the parts which constitute them, when the vehicle is in orbit. These mechanisms are most often ball bearings, but they can also include screw-nut systems and bearings of different types.

When spacecraft are launched, these various mechanisms are subjected to high mechanical stresses. In the case of ball bearings, the forces are transmitted from one ring to another passing through the balls. The presence or the creation of clearances during this phase would therefore lead to impacts between the balls and the rings, resulting in surface degradations which, in the long term, would lead to a malfunction then to the deterioration of the bearing.

In order to avoid any appearance of play in these mechanisms during launch, they are therefore usually applied a preload.

However, this preload is limited to a minimum value, to avoid unduly penalizing the mechanism under operational conditions. In fact, under these conditions, only a preload approximately 10 times lower than that which must be applied during launch. is really necessary. Thus, when the vehicle is in operational mode, any increase in the preload results in particular in an increase in the resistant torque of the mechanism, irregularities in operation linked to the fluctuation of this torque and a limitation of the service life.

In practice, the preload applied to the mechanisms is therefore the result of a compromise between the contradictory requirements which govern the launching and then the flight of the spacecraft.

Thus, in the particular case of a ball bearing, this bearing is dimensioned by determining the static forces representative of the vibrational loads which it undergoes during launching, then a minimum preload is applied to this bearing which is approximately equal to one third of these static forces. This preload must therefore be applied with good precision (approximately 20%), which requires the use of sensitive strain gauges and leads to a delicate, long and costly procedure. This method also requires interfaces which are not always compatible.

Given that a limited preload is applied to the mechanisms, the risks of creating games during launching are not completely eliminated in the presence of significant efforts.

Consequently, there is also the habit of interposing between the moving parts of the mechanisms, or between elements linked to these parts, a rigid stacking member through which part of the loads generated during launching are transmitted. When the spacecraft arrives in operational conditions, this rigid member must be broken by means of a pyrotechnic command.

Regardless of the spatial domain, the document Patent Abstracts, vol. 6, n ° 36 (M-115) [914], March 5, 1982 & JP-56 150614 describes a bearing in which an axial prestressing is applied by means of a prestressing spring and a spring made of a shape memory material placed end to end. At high speed, the heating of the bearing reduces the length of the spring in a shape memory material and, consequently, the axial prestress.

The subject of the present invention is a method and a device making it possible, in particular simple and non-binding, to release the preload applied during launch on mechanisms such as ball bearings on board space vehicles, after the arrival of these vehicles in operational mode, which allows, on the one hand, to make this unlimited preload and, consequently, to avoid any risk of degradation of the mechanisms during the launching phase and, on the other hand, to limit the preload in operational mode to what is strictly necessary, without the use of a rigid body of stacking combined with a pyrotechnic command is imperative.

According to the invention, this result is obtained by means of a device according to claim 1.

It is recalled that a shape memory material is a material which has two stable crystalline phases, respectively austenitic and martensitic, depending on whether its temperature is higher or lower than the structural transformation temperature characteristic of this material. When the material in martensitic phase is subjected to mechanical stress, it is plastically deformed and retains this deformation as long as the temperature remains below its structural transformation temperature. As soon as the temperature exceeds this threshold, the structure becomes austenitic again and the material returns to its initial shape.

The change in shape of the material caused by the crossing of the phase change temperature may in particular consist of an elongation. The member made of shape memory material then acts as a jack which is wedged before launching to a calculated dimension, for which a high preload is applied to the mechanism by the preload means. The heating of the material, which can be ensured either by a heating means such as an electrical resistance arranged near the member made of shape memory material, or by means of solar energy, lengthens this member, which has the effect of releasing the preload applied to the mechanism.

The invention also relates to a method of releasing a preload applied initially between moving parts of a mechanism on board a spacecraft when the latter is in operational mode, characterized in that it consists in controlling a change of shape of an organ into a shape memory material associated with this mechanism, so as to obtain a relaxation of the preload applied between the parts, by causing said organ to pass through a phase change temperature of the shape memory material .

• la figure 1 est une vue en coupe longitudinale d'une charnière ou articulation embarquée sur un véhicule spatial, comportant deux roulements à billes sur lesquels est appliquée une précharge axiale, ce montage étant muni d'un dispositif selon l'invention pour relâcher cette précharge, ce dispositif étant représenté dans son état initial inactif ; et
• la figure 2 est une vue comparable à la figure 1, dans laquelle le dispositif de relâchement de la précharge est représenté dans son état actionné.

An embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings in which:

• Figure 1 is a longitudinal sectional view of a hinge or articulation on board a spacecraft, comprising two ball bearings on which an axial preload is applied, this assembly being provided with a device according to the invention for releasing this preload , this device being shown in its initial inactive state; and
• Figure 2 is a view comparable to Figure 1, in which the preload release device is shown in its actuated state.

The joint shown in Figure 1 is, for example an articulation of a satellite or orbital station solar panel. This articulation comprises a shaft 10, assumed to be fixed in the embodiment shown, and an outer sleeve 12 mounted to rotate on the shaft 10 by means of two ball bearings 14 and 14 '.

Each of the bearings 14 and 14 'respectively comprises an inner ring 16, 16', an outer ring 18, 18 'and balls 20, 20' rolling between the rings.

The inner rings 16 and 16 ′ are mounted on the shaft 10 between a shoulder 10a formed on the latter and a tubular spacer 22 arranged coaxially around the shaft 10. The opposite end of this spacer 22 is placed opposite - screw of a nut 24 screwed onto a threaded end 10b of the shaft 10, a support washer 26 being interposed between the nut 24 and the spacer 22.

The nut 24 and the threaded end 10b of the shaft 10 thus constitute preload means by which an axial preload can be applied to the fixed inner rings 16 and 16 'of the bearings 14 and 14'. A tubular spacer 28 interposed between the rings 16 and 16 'ensures the transfer of this preload from one ring to the other.

The rotating outer rings 18 and 18 'are mounted in the outer sleeve 12 between a shoulder 12a and a nut 30 screwed into a reamed end 12b of the sleeve. A tubular spacer 32 is also interposed between the outer rings 18 and 18 '.

The structure which has just been described is conventional, with the exception of the tubular spacer 22, the support washer 26 usually being in direct contact with the inner ring 16 'of the bearing 14'.

According to the invention, an additional part is added to this structure, constituted by a second tubular spacer 34 which is produced in a shape memory material and placed coaxially inside the spacer 22 around a reduced diameter portion 10c of the shaft 10, on which the threaded end 10b is formed. More specifically, the tubular spacer 34 is placed between a shoulder 10d of the shaft 10 located in a plane close to the end face of the inner ring 16 'on which the spacer 22 is supported, and a shoulder 22a formed in the latter near the washer 26.

In the position illustrated in FIG. 1, which corresponds to the mounting position on the ground of the articulation, and which remains unchanged during the entire duration of the launch of the spacecraft on which this articulation is loaded, there is an axial clearance J1 between the spacer 34 and the shoulders 10d and 22a. Consequently, the preload obtained by tightening the nut 24, which is transmitted by the spacer 22 to the inner rings 16 'and 16 of the bearings 14' and 14, is not supported by the spacer 34 made of material to shape memory.

For the production of the tubular spacer 34, a suitable shape memory material is chosen, according to the conditions of use, taking into account in particular the temperature of phase change of the alloy. In no way limiting, mention may be made, among the shape memory materials which can be used in the invention, of iron-based alloys (Fe-C, Fe-Cr, Fe-Ni, etc.), copper alloys or of noble metals ( Cu-Zn, Cu-Sn, Cu-Al, Cu-Zn-Al, Cu-Zn-Al-Ni, Cu-Al-Ni, Ag-Cd, Au-Cd, ...), alloys based on titanium and / or Nickel (Ni-Ti, Ni-Ti-Fe, Ni-Al, ...) and certain pure metals (CO, Ti, Na).

In the embodiment illustrated in the figures, the heating of the tubular spacer 34 in shape memory material up to the transformation temperature of this material is obtained by means of a heating device constituted, for example, by a electrical resistance 36 wound directly around the spacer 34. Electrical conductors 38 make it possible to connect this electrical resistance 36 to a source of electric current (not shown) on board the vehicle, by means of a switch (not shown) open at launch.

When an order to close this switch is issued, the spacer 34 made of shape memory material is heated to a temperature higher than its structural transformation temperature. The structure of this material therefore becomes austenitic again, which has the effect of making it return to its initial shape which, in this case, corresponds to a greater length of the spacer 34.

As illustrated in FIG. 2, an adequate dimensioning of the components of the assembly has the effect, during the elongation of the spacer 34, of causing a permanent elongation of the part 10c of reduced diameter of the shaft 10, as a result of the coming into abutment of the spacer 34 on the shoulders 10d and 22a. Under these conditions, the preload initially applied to the inner rings of the bearings via the spacer 22 is released and a clearance J2 appears between the spacer 22 and the inner rings 16 and 16 'of the bearings.

In the embodiment illustrated in FIGS. 1 and 2, this clearance J2 is eliminated by placing, during assembly, at least one elastic washer 40, or any technically equivalent means, between the inner ring 16 'and the spacer 22. This washer elastic 40 maintains, under operational conditions, a slight axial preload which has the sole purpose of eliminating the clearance J2.

To facilitate the elongation of the reduced diameter part 10c of the shaft 10, this part 10c is advantageously machined in 10th over part of the length, to further reduce the section, as illustrated in the figures.

In an alternative embodiment not shown, the heating of the shape memory material is obtained without using the electrical energy of the satellite, by orienting the latter after it has been put into orbit, so that the material is then heated by the energy solar.

In an assembly opposite to that just described, that is to say in which a rotating shaft is mounted in a fixed sleeve by means of one or more bearings, a similar structure is used in order to initially apply an axial preload on the outer ring of the bearings.

Thanks to the device according to the invention, a relaxation of the preload applied initially on mechanisms such as bearings or screw-nut systems on board space vehicles is carried out by simple and space-saving means, as soon as these vehicles are in mode. operational. The risks of creating games in these mechanisms during launch are therefore eliminated since a substantially higher preload than before can then be applied. In addition, the removal of the limitation of this preload makes it possible to appreciably simplify the procedure for applying the preload, which requires only a precision. ± 100%. The need to interpose a rigid stacking part during launching and to break this part after launching by means of a pyrotechnic command, is also eliminated, which eliminates all the drawbacks inherent in this latter system. Finally, the elimination of the preload in operational mode, or the application of a very weak preload, very significantly increases the reliability of the mechanisms.

Furthermore, the advantages inherent in shape memory materials add to these advantages. Thus, the reliability of a mechanism thus controlled is total, its operation generates neither shock nor pollution, and its size is very small.

Of course, the invention is not limited to the embodiment which has just been described by way of example, but covers all its variants. As already observed, its application is not limited to ball bearings. In the same spirit, the structure of the preloading means can also be different from that which has been described, as well as the shape of the part made of shape memory material. Finally, the relative rotation between the two parts of the mechanism can be unlimited or, on the contrary, limited to angles less than 360 °.

Claims (10)

1. Apparatus for the release of a prestress or preload initially applied by prestressing or preloading means (24,10b) between the moving parts of a mechanism, said apparatus comprising a member (34) made from a shape memory material and acting on the prestress or preload means (24,10b) so as to control a release of the prestress or preload applied between said parts under the effect of a change to its shape, when clearing a phase change temperature of the shape memory material, characterized in that the mechanism is carried on a spacecraft and operated when the latter is in the operational mode and by the fact that the memory material member (34) does not form part of the prestressing means (24,10b), so that the preload initially applied between the parts is not supported by the memory material member (34).
2. Apparatus according to claim 1, characterized in that the mechanism comprises at least one bearing (14,14') having a fixed ring (16,16') and a rotary ring (18,18') between which are placed the rolling members (20,20'), the prestressing means (24,10b) acting on the fixed ring via a first tubular spacer (22), so as to initially apply an axial prestress to said fixed ring.
3. Apparatus according to claim 2, characterized in that the shape memory material member (34) is a second tubular spacer arranged coaxially to the first spacer (22), the clearing of the phase change temperature having the effect of elongating the second spacer and of transferring the prestress from the fixed ring to the second tubular spacer.
4. Apparatus according to any one of the preceding claims, characterized in that the prestressing means (24,10b) act on said parts via elastic means (40).
5. Apparatus according to claim 4, combined with either of the claims 2 and 3, characterized in that the elastic means (40) are interposed between the first tubular spacer (22) and the fixed ring (16b) of the bearing.
6. Apparatus according to any one of the claims 2,3 and 5, characterized in that the fixed ring is an inner ring (16,16') of the bearing mounted on a fixed spindle (10), the prestressing means incorporating a nut (24) screwed onto a threaded end (10b) of said spindle traversing the first tubular spacer (22).
7. Apparatus according to any one of the preceding claims, characterized in that it also comprises heating means (36) located in the vicinity of said shape memory material member (34).
8. Process for the release of a prestress initially applied between the moving parts of a mechanism carried on a space vehicle, when the latter is in the operational mode, characterized in that it comprises controlling a shape change of a shape memory material member (34) associated with said mechanism, so as to obtain a release of the prestress applied between the parts by making said member clear a phase change temperature of the shape memory material.
9. Process according to claim 8, characterized in that said member (34) is made to clear the phase change temperature of the shape memory material by heating said member with the aid of heating means (36) located in the vicinity thereof.
10. Process according to claim 8, characterized in that said member (34) is made to clear the phase change temperature of the shape memory material by heating the member by solar power.

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